1. Field of the Invention
The present invention relates to a liquid crystal display which is provided with a plurality of pixels including a liquid crystal element, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, liquid crystal displays of VA (Vertical Alignment)-mode for example, in which vertical alignment liquid crystal is used, is proposed as a display monitor such as a liquid crystal television, notebook PC, car navigation, and so on. It is known that the VA-mode can realize a large viewing angle compared with the TN (Twisted Nematic)-mode because of its liquid crystal molecule with negative dielectric anisotropy, i.e., a state in which the dielectric constant in the direction of the major axis of molecule is less than the dielectric constant in the direction of the minor axis.
However, the VA-mode liquid crystal display has a disadvantage of luminance displacement, that is, the luminance ratio at the time that a display screen is viewed from a front direction is different from that at the time of viewed from an oblique direction. FIG. 11 is a graph showing a relationship between the gray scale (0 to 255) of a video signal and the luminance ratio (ratio against a luminance over 255 levels of gray scale) in the VA-mode liquid crystal display. As shown by an arrow P101 in the graph, there is a big difference between the luminance characteristic as viewed from the front direction (indicated by the curved line “Ys (0°)” in graph) and the luminance characteristic as viewed from the direction of 45 degrees (indicated by the curved line “Ys (45°)” in graph). It proves that the luminance is displaced higher when seen from the direction of 45 degrees than seen from the front direction. Such phenomenon is called “whitish” “wash out”, or “color shift”, etc., and is considered as the greatest defect of the VA-mode liquid crystal display. To solve the whitish, it is proposed to divide a unit pixel into a plurality of sub pixels so as to set the threshold of each sub pixel different from each other (what is called a multi-pixel structure). Various types of multi-pixel structures have been proposed so far, and Japanese Patent Application Publication No. H02-12, U.S. Pat. No. 4,840,460 and Japanese Patent No. 3076938 disclose multi-pixel structures in which what is called HT (halftone-gray scale) method based on capacitance coupling is applied. In the multi-pixel structure shown in these patent documents, the potential difference between the plurality of sub pixels is determined based on capacitance ratio. More specifically, the multi-pixel structure includes such circuit configuration as shown in FIG. 12, in which a pixel 120 constituted from a sub pixel 120A and a sub pixel 120B are disposed in an area surrounded by a pair of data line D and gate line G that intersect with each other. In the multi-pixel structure of this type, the sub pixels 120A and 120B have liquid crystal elements 122A and 122B and auxiliary capacitive elements 123A and 123B, respectively, and they are connected to the source of a common thin film transistor (TFT) element 121 that is shared by the sub pixels 120A and 120B. The liquid crystal elements 122A and 122B and the auxiliary capacitive elements 123A and 123B are connected in parallel to the TFT element 121. However, a coupling capacitance Cc is provided in series between the TFT element 121 and the sub pixel 120B. The data line D is connected the drain of the TFT element 121, and the gate line G is connected to the gate of the TFT element 121. Further, the other ends of the auxiliary capacitive elements 123A and 123B are connected to a common auxiliary capacitance line CL which is shared thereby. In such multi-pixel structure with the above-mentioned circuit configuration, the potential difference between the sub pixel 120A and the sub pixel 120B is determined based on the magnitude of the coupling capacitance Cc, so that the ratio of luminance between the sub pixel 120A and the sub pixel 120B at a certain gray scale is determined.
Japanese Patent Application Publication No. 2003-255305 discloses another example of multi-pixel structure. In the multi-pixel structure, the pretilt angle of a liquid crystal molecule is different between a plurality of sub pixels so that the threshold may become different between the plurality of sub pixels.
FIG. 13 shows an example of relationship between the gray scale of a video signal and display mode of the respective sub pixels in the multi-pixel structure of related arts. Here, it is shown that in the course of increasing gray scale (increase in luminance) from zero level (black display level) to 255 level (white display level), a part of a pixel (one of the sub pixels) is increasing its luminance first, then the remaining portion of the pixel (the other of the sub pixels) follows. With such a multi-pixel structure, as shown by an arrow P102 of FIG. 11, for example, the curved line “Ym(45°)”, which indicates the luminance characteristic of the multi-pixel structure as viewed from the direction of 45 degrees, is more similar to the curved line “Ys (0°)” than the curved line “Ys (45°)”, which indicates the luminance characteristic of ordinary pixel structures as viewed from the direction of 45 degrees. Thus, it proves that the whitish phenomenon has been suppressed.
Other than the above-mentioned multi-pixel structures, it is also possible for an ordinary pixel structure as shown in FIG. 14 to obtain the same half-tone effect as with the multi-pixel structures by dividing a unit frame of display into a plurality of sub frames (for example, two sub frames) along the time axis and obtaining a desired luminance by separately expressing a high luminance and a low luminance using the two sub frames at different times. In this manner, the whitish phenomenon is suppressed.